Magnesium is a versatile metal with many commercial uses and can be worked by practically all known methods. Magnesium can be cast by sand, die, permanent mold, and precision investment methods; extruded into numerous shapes; and rolled (or wrought) into sheet, plate, or strip metal. Most end uses of magnesium require some degree of protection against corrosion and, in end uses which require paint, paint adhesion. Accordingly, a coating which improves the corrosion resistance and paint adhesion of magnesium is often applied to the metal prior to the final paint or other decorative finish is applied. Such coatings are referred to as pretreatments. Prior to the deposition of a pretreatment, however, it is important to sufficiently clean magnesium; otherwise, the poorly cleaned areas will become sources for poor paint adhesion and likely corrosion. In many cases, it has been determined that poor paint adhesion can be attributed to a poorly cleaned specimen, and not to the performance of the pretreatment itself.
In general, there are numerous cleaning methods and baths which have been used to clean magnesium. In their article, "Magnesium Finishing: Chemical Treatment and Coating Practices," Reese W. Murray and James E. Hillis disclose four general cleaning processes of magnesium: mechanical cleaning, solvent cleaning, alkaline cleaning, and acid pickling. This article mentions that these methods can be used singly or in combination. Table 6 of this article lists a variety of acid pickles used in cleaning magnesium, and the selected pickling agent depends on the manner by which magnesium is formed. Only chromic acid is listed as a pickling agent for all forms of magnesium, and an acetic-nitrate combination is disclosed as useful on wrought magnesium (but not castings) to remove mill scale. Mill scale consists chiefly of magnesium oxide and its hydrates, finely divided magnesium metal, and a carbon or carbonized oily lubricant. Mill scale is picked up by the rolled articles of magnesium during working and appears as scattered specs or imperfections in the rolled articles.
U.S. Pat. No. 2,302,939 to De Long discloses the use of either aliphatic carboxylic acid solutions or hydroxy aliphatic carboxylic acid solutions to remove mill scale from the surfaces of rolled articles of magnesium and magnesium-based alloys. Acetic, citric, and tartaric acids are disclosed as particularly effective for removing mill scale. This patent discloses that these organic acidic solutions attack mill scale much more rapidly than they do the magnesium metal, as opposed to the inorganic acids used prior to this patent. The treated magnesium article can then be coated with a protective finish such as a nitrate-chromate bath.
As can be inferred from the above, the selected cleaning method depends on the manner by which the magnesium was formed. The impurities formed during die casting of magnesium can be particularly troublesome and difficult to remove, especially when compared to mill scale. During die casting of magnesium, oxides and hydroxides of magnesium and alloying elements, such as aluminum, are formed on or near the surface of the castings. Furthermore, the lubricants used on the dies themselves are another source of impurities. Collectively, all of the impurities formed during the die casting process and remaining on the surface of the casting, including oxides and hydroxides of magnesium, aluminum, and other alloying elements and the impurities from the lubricants, will be referred to herein as "die casting impurities."
It has traditionally been believed that a relatively caustic (and typically toxic) cleaning agent is required to sufficiently remove die casting impurities, and that a relatively high degree of undesirable etching of the metal surface must be done in order to adequately clean the metal. A problem associated with the use of caustic baths is the disposal of such baths. For example, many sewer treatment authorities carefully regulate the types of baths that can be disposed of through sewers, and caustic acids, such as sulfuric acid, phosphoric acid, hydrofluoric acid, and nitric acid, are often forbidden.
Scott Fairchild, in his article "Technical Note No. 20--Surface Treatments," lists a number of acid pickling treatments for wrought magnesium alloys (Table 7A), for wrought or cast magnesium alloys (Table 7B), and for cast magnesium alloys (Table 7C). A mixture of acetic acid and sodium nitrate is mentioned in Table 7A for wrought magnesium alloys, while most of the pickling agents listed in Table 7C for cast magnesium alloys are more toxic and include sulfuric acid, nitric/sulfuric, chromic/nitric/hydrofluoric, phosphoric acid, and hydrofluoric/sulfuric.
Once cleaned, a pretreatment must then be applied to the magnesium die castings, as mentioned above. Baths containing hexavalent chromium have traditionally been used as coatings to improve the corrosion resistance and paint adhesion of magnesium die castings. Such baths have been used both as pretreatments for subsequent painting and as stand-alone coatings for corrosion protection. Chromium has been recognized as a carcinogen and as a cause of other environmental hazards, however. Accordingly, its use is sought to be minimized for the safety and protection of workers and others nearby. Furthermore, chromium baths often require a final rinse prior to the application of any decorative coating. The disposal of the final rinse bath, as well as the chromium bath itself, is costly.